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Modal analysis query on modal masses

  1. Yes

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  2. No

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  3. Inapplicable

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  1. Jun 14, 2010 #1
    Hello guys,
    I have a small query on structural dynamics. Please help me out.

    -Is it necessary that the modal masses of higher modes of a structure are lesser than that of the lower modes? I carried out a modal analysis of a dam considering 10 modes of vibration applying vibration in the x-direction.

    The modal masses for the 10 modes obtained for the structure in the x direction are as follows:
    1 2.94E+06
    2 1.51E+06
    3 46076.8
    4 393957
    5 46973.9
    6 89936
    7 18464.5
    8 80810.4
    9 62870.3
    10 57793.2

    Similarly for y direction,
    1 67858
    2 681348
    3 3.00E+06
    4 317113
    5 230273
    6 642.784
    7 450142
    8 23062
    9 108110
    10 23772

    We can notice clearly that for the x direction the modal mass for the mode no. 3 is lesser than that of mode no. 4. There are other instances as well that we can observe. In the third mode shape there was much of a vertical movement and therefore it can be argued that the modal mass for the y direction is high in the third mode. But I have a doubt if I will be correct in putting such a logic forward.
    I used a finite element software to arrive to these results. Is my analysis incorrect?
    Please help.
    Thanks in advance.
  2. jcsd
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